1. Field of the Invention
This invention relates generally to improvements in apparatus for the measurement of fluid flow and, more particularly, to a new and improved apparatus for measuring gas flow in a respiratory therapy system for the treatment of sleep apnea and other breathing disorders.
2. Description of the Prior Art
There are numerous applications in modern engineering practice that require the measurement of fluid flow. Over time, tubes of various sizes and shapes have been designed and constructed for this purpose.
The first description of a tube used to measure fluid pressures for velocity determinations is credited to Henri Pitot, and tubes for this purpose have frequently been named after him. Today, there are many different arrangements and geometrical shapes of Pitot tubes adapted for a wide variety of applications.
A typical Pitot tube arrangement consists of a cylindrical tube with an open end located in the fluid flow stream and pointed upstream to measure stagnation or impact pressure (where the stream is decelerated to zero velocity), plus one or more sidewall taps in the flow conduit to measure local static pressure in the moving stream. The difference between the impact pressure and the static pressure is a function of the velocity of the flow stream.
Upstream disturbances in the fluid flow stream have a tendency to cause large errors in the flow measurement, in part because of the turbulence generated and its effect on the static pressure measurement. A calming section of a least several pipe diameters or more is often utilized to obtain accurate measurements. Pulsating flow also can have an adverse effect on flow measurement accuracy, to the extent that damping mechanisms are sometimes used to avoid significant measurement errors.
Flow measuring devices are commonly used in a wide variety of modern medical systems. One type of such system is a respirator used for the treatment of obstructive sleep apnea.
Obstructive sleep apnea is a sleeping disorder characterized by relaxation of the airway including the genioglossus throat muscle during sleep. When this occurs the relaxed muscle can partially or completely block the patient's airway. Partial blockage can result in snoring or hypopnea. Complete blockage results in obstructive sleep apnea.
When complete blockage occurs, the patient's inhalation efforts do not result in the intake of air and the patient becomes oxygen deprived. In reaction, the patient begins to awaken. Upon reaching a nearly awakened state, the genioglossus muscle resumes normal tension which clears the airway and allows inhalation to occur. The patient then falls back into a deeper sleep whereupon the genioglossus muscle again relaxes in the apneic cycle repeats. In consequence, the patient does not achieve a fully relaxed deep sleep session because of the repetitive arousal to a nearly awakened state. People with obstructive sleep apnea are continually tired even after an apparently normal night's sleep.
In order to treat obstructive sleep apnea, a system of continuous positive airway pressure (CPAP) has been devised in which a prescribed level of positive airway pressure is continuously imposed on the patient's airway. The presence of such positive pressure provides a pressure splint to the airway in order to offset the negative inspiratory pressure that can draw the relaxed airway tissues into an occlusive state.
The most desired device for achieving a positive airway connection is the use of a nasal pillow such as that disclosed in U.S. Pat. No. 4,782,832, hereby incorporated by reference. The nasal pillow seals with the patient's nares and imposes the positive airway pressure by way of the nasal passages. The nasal pillow also includes a small vent for continuously exhausting a small amount of air in order prevent carbon dioxide and moisture accumulation.
In the CPAP system, the patient must exhale against the prescribed positive pressure. This can result in patient discomfort, especially at the higher pressure levels. Because of this problem, the so-called bi-level positive airway pressure (BiPAP) system has been developed in which the pressure is lowered during the exhalation phase of the respiratory cycle.
In a BiPAP system, air flow to the patient is measured by a flow measuring device pneumatically coupled to the nasal pillow. The measuring device is connected to a flow sensor with a transducer to produce an electrical signal representative of the air flow delivered to the patient. The signal is used by electronic circuitry to control the pressure of the respiratory gas being delivered to the patient. A pressure sensor also is used to provide the circuitry with a signal representative of the pressure being delivered to the patient.
These types of respirator systems can experience turbulent and pulsating flow in the delivered stream of respiratory gas. Turbulence is often the result of bends in the flow path and insufficient space to provide for calming sections to return to laminar flow. Additionally, under certain conditions, the direction of flow can reverse itself, causing problems for uni-directional flow sensors.
It is apparent that accurate measurement of the air flow provided to the patient is an important feature in a respirator system. Ideally, the measurement should accurately reflect the dynamics in a bi-directional flow system and should be accomplished in a manner that provides a high signal-to-noise ratio in both turbulent and laminar flow regimes. The flow measurement also should have linear characteristics with proportionality to the total flow in the system.
Design constraints sometimes require that the flow measurement be determined in a limited space with negligible resultant pressure drop. It is also sometimes desirable that the flow measurement apparatus be incorporated within other parts of the system to reduce the total number of parts. This should be accomplished in a manner which promotes ease of manufacture at low cost.
The traditional Pitot tube employed in previous respirator designs used a right angled tube with an opening directed toward the incoming flow stream and a second right angled tube with an opening directed away from the flow stream. A side flow was developed when these two tubes were immersed in a conduit containing the air flow stream. This side flow was routed to a sensor/transducer. The sensor developed a signal which in turn was conditioned to accurately reflect the total flow output of the system.
While these existing respirator flow measuring devices have served their purpose, there remains a continuing desire for further improvement therein. In particular, a need exists for an improved flow measuring device which enables more accurate and reliable measurements to be made in turbulent or pulsating flow, with high signal-to-noise ratio, and which works well with flow in either direction. The present invention fulfills all of these needs.